Its going to be awfully hard to gauge her true sincerity if she realizes that you could withdraw your sponsorship and have her sent home. That is probably her main obsession. She may actually dislike you and still put on a show of 'remorse' to avoid leaving the USA.
Just what are her true feelings? She could be faking every word she utters and you wouldn't know.
All you know is that she cheated, wouldn't have confessed and probably would have continued having sex with OM had you not found out. So do you take a huge risk and forgive?
I have been exactly where you are that was 13 years ago. Since then we had kids and she has had other affairs
The post from stepping up says it all. If she is addicted to the attention and validation, then this may the first in a number of affairs.
If it were me I would ask one question. Did she use a condom with the OM or did she expose both of you to the risk of disease and pregnancy, and bring home her lovers issue to your marital bed. If she did not use protection, then she would be on that long flight home to wherever she came from.